Automobiles emit hydrocarbons, nitrogen oxides, carbon monoxide and carbon dioxide as a result of the combustion process. Automobile emissions are said to be a significant contributor to pollution. In order to reduce and/or eliminate such emissions automobile manufacturers have attempted to utilize alternative transportation fuels and/or alternative sources of power, such as, for example, fuel cells. Generally, fuel cells generate electricity by electrochemically combining across an ion-conducting electrolyte a fuel, such as hydrogen, carbon monoxide, or a hydrocarbon, and an oxidant, such as air or oxygen.
A fuel cell system typically includes a “stack” of individual fuel cells that are electrically interconnected in a series configuration. Thus, the number of cells in the stack, i.e., the number of cells connected together in series, determines the voltage that is produced by the stack. Each of the individual fuel cells within the stack produces a voltage that varies dependent at least in part upon the current being drawn from that cell and/or the stack. The voltage produced by a typical single cell varies from an open circuit voltage, such as, for example, approximately 1.0 Volts (V) at low or zero current loads to a lower limit, such as, for example, approximately 0.7 V, under high current loads. If the voltage produced by a cell drops below a minimum threshold, such as, for example, 0.6 V, an undervoltage condition exists that may result in damage to the cell, such as, for example, cell oxidation.
Since the voltage produced by each cell varies dependent at least in part upon the current load upon the cell, the voltage produced by the stack also varies dependent at least in part upon the current load. More particularly, due to the series interconnection of the cells in the stack, the variation in the voltage produced by the cells is cumulative, i.e., the stack voltage will vary in a manner that reflects the sum of the voltage variations of the individual cells within the stack. This cumulative effect on the stack voltage can be relatively substantial. For example, the voltage produced by a fuel cell having sixty cells may vary from approximately sixty volts to approximately forty-two volts.
Most electrical systems are designed to operate with a supply voltage that falls within a predetermined range. As described above, the voltage produced by a fuel cell stack may vary substantially. Thus, if a fuel cell system is to be used as a power source for such an electrical system the stack voltage must typically be regulated by a voltage regulating device or devices to ensure the stack voltage supplied to the electrical system remains within the voltage range required by the electrical system, independent of the voltage produced by the stack. As the amount of variation in the voltage produced by the stack increases a correspondingly greater amount of regulation is required in order to provide a supply voltage to the electrical system that is within the specified range. In order to provide adequate regulation of such a widely-varying voltage, voltage regulation or control devices that are relatively complex, costly, sizeable, and power consuming are required.
Therefore, what is needed in the art is a fuel cell system that substantially reduces damage and/or oxidation to the cells, such as, for example, due to an under voltage condition.
Furthermore, what is needed in the art is a method and apparatus that controls the output voltage of a fuel cell system while also controlling the operation of the fuel cell such that the fuel cell operates with improved efficiency relative to unregulated operation.
Moreover, what is needed in the art is a fuel cell system that generates a controlled and/or regulated output voltage.